1. Field of the Invention
The present invention relates to an image pickup device having a sensitivity wavelength range which includes near infrared wavelengths (short wavelength infrared (SWIR)), that is, night glow, among electromagnetic waves coming to the earth from the universe, and also relates to a vision enhancement apparatus, a night-vision apparatus, a navigation supporting device, and a monitoring apparatus, each of which uses the image pickup apparatus described above.
2. Description of the Background Art
(I) Night glow: Various types of electromagnetic waves (light) have come to the earth from the universe, and for example, radio astronomy that studies the form of the universe by observing x-rays coming from the universe has been well known. Among the electromagnetic waves coming to the earth from the universe, since the detection of the SWIR band does not require a large and expensive apparatus, intensive attention has been paid to the SWIR band in recent years, and many reports thereon have been published. For example, there has been a report in which observation results of the SWIR spectrum coming from the universe are described, and in which a spectrum having a peak at a wavelength between 1.4 to 1.9 μm is disclosed (Vatsia, Mirshri, L. “Atmospheric Optical Environment”, Research and Development Technical Report ECOM-7023, September (1972)). In addition, for example, a night-vision device using an InGaAs photodetector (In atom:Ga atom=0.53:0.47) lattice-matched with InP has been disclosed (Marchall J. Cohen, “Near-IR imaging cameras operate at room temperature”, LASER FOCUS WORLD, June 1993, p. 109 (Seniors Unlimited)). In this case, as shown in FIG. 24, an atomic ratio In/Ga is set to 0.53/0.47 in order to enable the lattice parameter to match with that of an InP substrate, and hence the sensing limit (threshold sensitivity) of the photodetector at a long wavelength side is approximately 1.7 μm. The structure of the photodetector shown in FIG. 24 will be particularly described in a comparative example. Hereinafter, light in the SWIR band which has come to the earth from the universe is called the night glow or the night glow in the SWIR band, or is simply called the SWIR band light.
(II) Compound semiconductor photodetector: In order to detect light having a wavelength longer than that detected by the above InGaAs photodetector (absorbable wavelength at a long wavelength side is not so long), a photodetector has been proposed as shown in FIG. 25 in which a graded layer having a graded composition or a step buffering layer is provided on InP for lattice matching, and the In composition is gradually increased while the lattice matching is maintained (Japanese Unexamined Patent Application Publication No. 2002-373999). In this case, since the atomic ratio In/Ga is set larger than that described above, the lattice parameter is gradually increased from an InP substrata to an absorption layer, and hence the sensitivity of the absorption layer (In/Ga=0.8/0.2) is enhanced to a longer wavelength side. The structure of the photodetector shown in FIG. 25 will also be particularly described in a comparative example.
(III) Night-vision apparatus: In recent years, a night-vision apparatus using light in a long wavelength region has also been proposed although it is not directly related to the technique described above. For example, there have been proposed an apparatus supporting vision enhancement of a backward field of view of an automobile, in which by irradiating an object including a person with infrared light, reflection thereof is picked up by an infrared camera (Japanese Unexamined Patent Application Publication No. 2004-350228); an automobile night-vision apparatus configured in combination of near infrared light-emitting diodes (LED) and an image pickup device, which is similar to the device described above (Japanese Unexamined Patent Application Publication No. 2002-274258); a vision device configured in combination of two wavelength regions, that is, infrared and near infrared regions (Japanese Unexamined Patent Application Publication No. 9-37147); and an in-vehicle image pickup device which absorbs light in a 1.5 μm band using InGaAs photodetectors (Japanese Unexamined Patent Application Publication No. 7-302928).
Problems of the techniques described above may be summarized as follows.
(A1) Compound semiconductor photodetector: Since noise and dark current are large, and in addition, many dark spots are present, a clear image cannot be obtained. In particular, as the temperature is increased, the image definition is considerably degraded.
(A2) Night-vision apparatus: A light source of infrared, near infrared, or the like is necessary. In addition, the device is complicated, and as a result, the cost thereof is increased. Furthermore, the image definition is not sufficient.
The problems of the above (A1) occur since a compound semiconductor which can absorb light in a longer wavelength region cannot be well formed to have good crystal quality. In addition, since having serious influences on social and economical situations, the problems of the above (A2) must be urgently solved. The present invention first overcomes the problems of the above (A2), that is, the present invention provides an image pickup device and a vision enhancement apparatus, which can obtain a clear image, although having a simple structure. Furthermore, the present invention provides an image pickup device having an absorption layer, which solves the problems of the above (A1), that is, which can reliably obtain a clear image while the noise and dark current are reduced, and in addition to that, by using this image pickup device, the present invention provides a vision enhancement apparatus, a night-vision apparatus, a navigation supporting apparatus, and a monitoring apparatus. In general, it is roughly classified such that the vision enhancement device is an device mounted in a vehicle (an automobile or the like) to enhance a field of view of a driver for safety improvement; the night-vision apparatus is a person-portable apparatus; the navigation supporting apparatus is an apparatus which is mounted in a vessel and which supports recognition of an object by some sort of means; and the monitoring apparatus is an apparatus placed at a predetermined position to monitor an object; however, the classification described above is not strict. Accordingly, the vision enhancement apparatus, the night-vision apparatus, the navigation supporting apparatus, and the monitoring apparatus can all be regarded as an apparatus including an optical device to which the above image pickup device is applied.